Analysis of Bull Proofs for Intermediate Optimum Traits - CGIL

sire’s proof on the desirability scale could be examined. The underlying hypothesis of this
approach is that the publication of proofs on the tendency scale alone would suffice if bulls with
proofs near the breed average showed a consistently desirable pattern for producing desirable
daughters.
Results & Discussion
Bull proofs aim to reflect the expected average performance of future daughters assuming a
continuous linear scale. For type traits with an intermediate optimum that are recorded on a 9point
linear scale, previous research found various bulls to have desirable daughter averages
near the optimum score of 5 based mainly on daughters at each extreme of the linear scale
rather than a majority at the ideal mid-score value. Computing genetic evaluations using these
data yield bull proofs near the ideal of zero (meaning no clear tendency towards either extreme)
and therefore suggest these bulls are breed improvers. For these reason, in 1997, it was
decided to use the same classification data, but on a folded scale of 1 to 5, to produce genetic
evaluations on a desirability scale, published as Set of Rear Legs and Pin Setting.
Figures 1 and 2 show a clear relationship between the percentage of daughters that have an
ideal score of 5 and the sire’s proof for Set of Rear Legs and Pin Setting, respectively, based on
bulls that have a proof on the tendency scale that is near breed average. Daughters of bulls
that are close to average for Rear Leg Side View and have a Set of Rear Legs proof of less than
-10 have about a 20% probability of having an ideal score of ‘5’ for Rear Leg Side View. In
contrast, daughters of bulls with an average proof for Rear Leg Side View and are at least +10
for Set of Rear Legs have more than 40% chance of having a score of 5. Similar results were
observed for Rump Angle/Pin Setting in Figure 2 with the spread in likelihood of having an ideal
daughter being from 30% at -10 to 50% at +10 for Pin Setting.
The standard deviation of daughter scores for bulls at opposite ends in their proof for Set of
Rear Legs (and Pin Setting) also has a clear trend based on bulls that have a proof near breed
average for the proof on the tendency scale. Basically, a higher standard deviation of daughter
scores for negative proofs on the desirability scale reflect that these bulls have a large variation
in how their daughters are scoring. Less variability in daughter scores for highly rated bulls on
the desirability scale indicate that the system is appropriately identifying bulls that are producing
daughters that consistently have the ideal phenotype.
Conclusion and Recommendation
The results of this analysis, as presented in Figures 1 and 2, clearly show that there are bulls
evaluated as ideal (i.e.: near zero) on the tendency scale that are inappropriately considered as
siring daughters near the ideal rating of 5. Therefore, the proof information on both the
tendency and desirability scales are required for a proper interpretation of bull proofs for these
intermediate optimum type traits. While this analysis was done only for the Holstein breed there
is no reason to assume that a similar result would not also occur for other breeds. It is
recommended that the current procedure for calculating proofs on both the tendency and
desirability scales be continued.